BiCh2-based superconductors (Ch: S, Se) are a new series of layered superconductor. However, mechanisms for the emergence of superconductivity in BiCh2-based superconductors have not been clarified. In this study, we have investigated crystal structu
re of two series of optimally-doped BiCh2-based superconductors, Ce1-xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1-ySey)2, using powder synchrotron x-ray diffraction in order to reveal the relationship between crystal structure and superconducting properties of the BiCh2-based family. We have found that an enhancement of in-plane chemical pressure would commonly induce bulk superconductivity in both systems. Furthermore, we have revealed that superconducting transition temperature for REO0.5F0.5BiCh2 superconductors could universally be determined by degree of in-plane chemical pressure.
We have systematically investigated the crystal structure, magnetic susceptibility, and electrical resistivity of the BiS2-based superconductor LaO0.5F0.5Bi(S1-xSex)2 (x = 0 - 0.7). With expanding lattice volume by Se substitution, bulk superconducti
vity was induced for x > 0.2, and the highest Tc of 3.8 K was observed in x = 0.5 (LaO0.5F0.5BiSSe). Metallic conductivity was observed for x > 0.3 in the resistivity measurement, whereas semiconducting-like behavior was observed for x < 0.2. The induction of bulk superconductivity by the partial substitution of S by Se in the LaO0.5F0.5BiS2 superconductor should be positively linked to the enhancement of metallic conductivity.
High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc
) of Tconset = 11.1 and Tczero = 8.5 K in the electrical resistivity measurements and Tconset = 11.5 and Tcirr = 9.4 K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.
We investigated the crystal structure and superconducting properties of As-grown and high-pressure-annealed PrO0.5F0.5BiS2. We found that the high-pressure annealing generates uniaxial lattice contraction along the c axis. Both As-grown and high-pres
sure-annealed PrO0.5F0.5BiS2 show bulk superconductivity. The Tc of PrO0.5F0.5BiS2 is clearly enhanced from Tczero = 3.6 K to Tczero = 5.5 K by high-pressure annealing. Unexpectedly, the semiconducting characteristics is relatively enhanced by high-pressure annealing. Namely, we assume that the enhancement of Tc can not be understood by an increase of electron carriers. Having considered these facts, we conclude that the enhancement of Tc correlates with uniaxial lattice contraction along the c axis in PrO0.5F0.5BiS2.
Correlation between crystal structure and superconducting properties of the BiS2-based superconductor LaO0.5F0.5BiS2 was investigated. We have prepared LaO0.5F0.5BiS2 polycrystalline samples with various lattice constants. It was found that the annea
ling the sample under high pressure generated uniaxial strain along the c axis. Further, the highly-strained sample showed higher superconducting properties. We concluded that the uniaxial strain along the c axis was positively linked with the enhancement of superconductivity in the LaO1-xFxBiS2 system.
Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors pos
sess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.
We have established a plot of the anion height dependence of Tc for the typical Fe-based superconductors. The plot appeared a symmetric curve with a peak around 1.38 A. Both data at ambient pressure and under high pressure obeyed the unique curve. Th
is plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and search for the new Fe-based superconductors with higher Tc.
We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature Tc was observed with the subs
titution of S or Te for Se; the Tc increased with S substitution by up to 20 %, and also increased with Te substitution up to 75 %. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine Tc in the FeSe system.
We have successfully synthesized a new superconducting phase of FeTe1-xSx with a PbO-type structure. It has the simplest crystal structure in iron-based superconductors. Superconducting transition temperature is about 10 K at x = 0.2. The upper criti
cal field Hc2 was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic elements, this material is a candidate for applications and will activate more and more research on iron-based superconductor.
Tetragonal FeSe is a superconductor with a transition temperature Tc of 8 K and shows a huge enhancement of Tc with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transitio
n. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.
